If you think the answer is never, you're wrong.
Here's the checklist (I'm big on checklists, have you noticed?)
1. Do I want to hear from you when you are ready to query? YES
What: send query. (don't send an email asking if it's ok to query; it is)
2. Do I want to hear from you if you didn't get a reply to your query in 30 days: YES
What: send the query again (don't ask if I got it, just send)
3. Do I want to hear from you when I ask you for a full: YES
What: .doc attachment (I acknowledge receipt of all fulls)
4. Do I want to hear from you IF you haven't heard back on a full in the amount of time listed in the acknowledgement: YES
What: "Dear Snookums, what's up with the hot novel I sent you on (date)"
5. Do I want to hear from you IF you've gotten an offer of representation and I'm reading your full: YES
How: email promptly please.
What: Let me know you're considering an offer, and how much time you have for your decision. I like to know who's offering too.
That's the list. Notice what's NOT on there:
1. You revised the manuscript
2. You revised the query
3. You left something off the query
4. You've got a question about what I want to see (the answer is YES, I want to read your query)
5. You got my request for a full and you'll get back to me when you're back from Pluto, vacation, etc.
Does this help?
I'll add to the list if you post your questions in the comment section.
And please note, this applies ONLY to people who are at the query stage. Hot prospects and clients get a whole new set of guidelines, mostly involving all-night liquor stores and speed dial.